Purification of magnesium



Patented Apr. 1'0, 1945 Q 7 PURIFICATION OF MAGNESIUM Philip T. Stroupand George F. Sager, New- Kensington, Pa., assignors to Aluminum Companyof America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.Application March 30, 1942,

. Serial No. 436,765

7 Claims. (01. 75-47) This invention relates to the removal of iron frommagnesium and it concerns more particularlythe removal of iron withtheaid of zirco- The term "magnesium as used-herein and in the appendedclaims will be understood to refer not only to magnesium itself but alsoto alloys suspel ion throughout the magnesium matrix,

and this finely dispersed iron usually causesa marked reduction in theresistance of magnesium to corrosive attack. a

In our copending application Serial No. 436,764 filed Mar. 30, 1942, forPurification of magnesium, a method is described which employs zirconiumchloride for removing iron from molten magnesium. The zirconium chlorideintroduced into the molten metal appears to react with magnesium,releasing zirconium into the melt and this zirconium, without thenecessity of any further treatment oi the melt, removes iron from themagnesium in a manner more satisfactory than zirconium, introduced intomagnesium by other methods.

The object of the present invention is to remove iron from magnesium.More particularly,

.the object is to make the aforesaid method of removing iron from moltenmagnesium with the aid of zirconium chloride as economical andasconvenientin actual plant practice as possible. For this purpose, wehave sought to provide a way whereby a charge of zirconium chloride canbe in th magnesium to form a magnesium-zirconium alloy, settles to thebottom of the melt, carrying iron also to the bottom it the magnesiumcontains iron. The supernatant magnesium, or the supernatantmagnesium-zirconium alloy which may have been formed, then can bedecanted or-otherwise removed from the crucible, leaving as a residue aheel of magnesium rich inzirconium.

The magnesium-zirconium alloy, which may be either the heel or thesupernatant alloy separated from the zirconium-rich heel, canbe'utilized as an addition alloy to introduce zirconium into subsequentmelts of magnesium to remove iron from them. It is necessary, however,that enough of the addition alloy from the initial melt be added toprovide the subsequent melt with at least 0.1

per cent of zirconium since about that amount of zirconium is requiredto effect a satisfactory removal of iron. 'Since a greater amount ofzirconium generally is present in the heel than in the decantedmagnesium-zirconium alloy, we find it more convenient in the practice ofour invention to utilize the heel as the addition alloy for introducingzirconium and removing iron in thesubsequent melts than to use thedecanted alloy for that purpose. Accordingly, the-invention will bedescribed as embodied in the use of this heel for introducing zirconiumand'removing iron from subsequent melts of magnesium, but it will beunderstood that the decanted purifled magnesium, when it containsenoughv zirconium after .the treatment with zirconium chloride toprovide a subsequent melt with 0.1 per cent or more of zirconium, can beused in the same way. as the heeland for the same purpose.

When the zirconium-rich heel from the initial melt is introduced intoanother body of molten magnesium to provide 0.1 per cent or mo;'e ofutilized to remove iron from more than one melt of magnesium. J

Our present invention is predicated upon our discovery that an alloy ofmagnesium .containing zirconium derived by reaction. of zirconiumchloride with magnesium can itself be introduced into a melt ofmagnesium to provide that melt withzirconium eflective for removingiron.

We have found that when 0.25 per cent or more ma preferably when from0.25 to 5 percent of zirconium chloride is charged into molten magnesiumin the practice of the invention described and claimed in the aforesaidapplication for Letters Patent. the zirconium thereby introduced, exceptfor a small amount which may be dissolved zirconium to that body ofmolten magnesium, the zirconium precipitates and in from about minuteup, to 'about' 15 minutes, depending upon the a size of the melt, removeiron in that body of magnesium, forming another heel which may itself beused to remove iron and to form a heel in yet another melt of magnesium.Accordingly, our present invention also embraces utilizing the heelformed in each 01' a series of melts-to introduce zirconium and removeiron from the magnesium in the next succeeding melt.

Following treatment ot--the initial melt or of each succeeding melt ofmagnesium in the prac-- tice of our invention, the precipitatedzirconium-v rich heel and the, purified magnesium, which-of course, maycontain some zirconium, can easily be separated. We prefer to decant theupper purified portion of the melt from the crucible, leaving the lowerprecipitate-containing portion of the melt, or the heel, as a residueready to be mixed into the next melt to be treated for removal of iron.Instead, however, the heel may,

be removed from the crucible by methods known to the art, leaving thepurified magnesium, instead of the heel, in the crucible.

In utilizing the heel from one melt for purifying the following melt, itis possible to transfer the heel from the crucible in which it is formedto another crucible already containing the molten -magnesium to bepurified, and thereby to introducezirconium into that melt. We find itmore convenient, however, 'to leave the heel in the bottom of thecrucible after the magnesium is decanted and to charge subsequent meltsinto that crucible and over the heel. Each melt, properly protected by asuitable flux or atmosphere, pref erably is agitated to stir the heelinto the magnesium; and the heel re-iormed in each melt by the settlingzirconium and iron is left in the crucible when the purified magnesiumis removed and is used to purify the succeeding melt.

Instead of keeping the heel in the molten state after the purifiedmagnesium is poured from the crucible, the heel may be allowed tosolidify and then either melted down with the next charge of magnesiumor added after the next charge is molten. In large scale meltingpractice, however, it is more convenient to have the heel already in themolten condition when the next charge of magnesium is introduced intothe crucible.

Since iron as well a zirconium is contained in the heel which formsfollowing treatment of each body of molten magnesium, the introductionof each heel into a subsequent melt introduces iron as well as zirconiuminto that melt. This iron and the iron already in the magnesium thensettle out with zirconium. During treatment of a succession of meltswith a single charge of zirconium chloride, the iron content of the heelincreases as the iron removed from each successive melt of ma nesiumbecomes added to the heel. Eventually the iron content in the heelbecomes so great that zirconium in the heel is incapable of settlingboth the iron impurity in the magnesium being treated and the ironintroduced from the heel. At this point, the iron content is usuallyabout 1.5 per cent of the weight of the heel and it is necessary eitherthat the heel be augmented with more zirconium introduced as zirconiumchloride in order to prolong the effectiveness of the heel for removingiron or that the heel be discarded and. a new heel formed with zirconiumchloride. Even without the addition of supplementary amounts ofzirconium chloride, however, a single heel formed in an initial melt ofmagnesium by the introduction of zirconium chloride equivalent to 0.25to 5 per cent of the total weight of the initial melt normally can beused to remove iron from a succession of up to about 12 subsequentmelts. An even greater number of melts of magnesium may be treated forthe removal of iron it fresh amounts of zirconium chloride are addedfrom time to time or if a greater amount of zirconium chlorde originallyis added to the initial melt to form the heel.

The operation of our invention described and claimed herein is efiectivefor the removal of iron impurity from both magnesium itself and frommagnesium containing the usual amount of common alloying elements. Forexample, iron may be removed from magnesium base alloys containing from1 to 15 per cent aluminum. or 0.2 to 10 per cent zinc, or 0.1 to 2.5 percent manganese, or 0.5 to 8 per cent tin, or combinations of theseelements. Other elements also may be present as alloying ingredient inmagnesium treated by the method of our invention.

"mens were cast in sand molds from each melt and these were analyzed todetermine. their iron contents. The first melt then was charged withzirconium chloride in briquette form in an amount and the molten alloywas poured off, leaving a heel containing magnesium, zirconium, and,iron at the bottom of the crucible. The decanted metal then wasanalyzed, ,and its iron content was found to have been diminished by thetreatment with zirconium from 0.028 to 0.005 per cent.

The heel left after the pouring of the first melt then was admixed withthe second melt and allowed to settle. The purified magnesium then wasdecanted and test specimens cast in sand from the decanted portion wereanalyzed-to determine the extent of the iron reduction by treatment withthe zirconium-rich heel. Thereupon, the heel was admixed with a thirdmelt to remove it iron and the heel formed in this third melt wascharged into the fourth melt. Altogether, the heel was chargedsuccessively from one portion to the next until all eight melts had beentreated. The following table shows the amount of iron in each meltbefore and after the treatment with zirconium.

Mon Amount of Amount of iron before iron af ter treatment treatment e 1Treated with 2 ZrCli P r f Per (05 2 Treated with heel from No. l. 022.009 3 Treated with lice] from No. 2. .031 .006 4 Treated with heel fromNo. 3. .017 .005 5 Treated with heel from No. 4. .016 .008 f i Treatedwith heel from No. 5. .012 .007 I Treated with heel from No. 6. .027.015 S Treated with heel from No. 7. 020 .011

This example will be understood to be illustrative of the practice 01'our invention and it will be appreciated that modificationsw'hich willappear to those skilled in the art will be within the scope or ourinvention.

We claim:

1. The method of removing iron impurity from a melt of magnesium whichcomprises mixing said melt with magnesium containing zirconiumpreviously provided therein by reaction of zirconium chloride withmagnesium, permitting the zirconium and iron in said melt to ettle tothe bottom of the container, and separating from each other the upperpurified portion and the lower precipitate-containing portion of theresultant melt.

2. The method of removing iron impurity from a melt of magnesium whichcomprises mixing said melt with magnesium containing zirconiumpreviously provided therein by reaction of at least about 0.25-per centby weight of zirconium chloride with magnesium, permitting the zirconiumand iron to settle to the bottom of the container, and separating fromeach other the upper purified portion and the lowerprecipitate-containing portion of the resultant melt.

3. The method ofremoving iron impurity from a melt of magnesium whichcomprises mixing said melt with magnesium containing zirconiumpreviously provided therein by reaction of between about 0.25 and 5.0per cent by'weight of zirconium chloride and subsequent settling of ironand of the zirconium'with magnesium, permitting said melt of magnesiumto stand for a time between about one-half minute and about 15 minuteswhile precipitation of zirconium and iron to the-bottom of the containertakes place, and decanting the purified magnesium from the precipitatedzirconium and iron.

4. The method of removing iron impurity from a melt of magnesium whichcomprises mixing said melt with an initial body of magnesiumcontainingzirconium derived by reaction therein of zirconium chloride withmagnesium, the zirconium content of said initial body of magnesium beingsuificientto provide said melt of magnesium with at least 0.1 per centof zirconium. permitting the zirconium and iron in said melt to settleto the bottom of the container, and separating from each other the upperpurified portion and the lower precipitate-containing portion of theresultant melt.

5. The method of removing with a heel derived from an initial body ofmagiron from a melt of magnesium which comprises mixing said melt nesiumcontaining zirconium provided by reaction therein or zirconium chloridewith mag-.

nesium, the zirconium content of said heel being suflicient to introduceat least 0.1 per cent of zirconium into said melt, permitting thezirconium and iron in said melt to settle to the bottom of thecontainer, and separating from each other the upper purified portion andthe lower precipi- 1:.

tate-containing portion of the resultant melt.

6. The method of removing iron impurity from a succession of melts ofmagnesium which comprises introducing zirconium chloride in an initialmelt of magnesium in an amount suificient to remove iron and to form aheel containing zirconium in said initial body of magnesium, separatingsaid heel from said initial melt, charging the heel into a second meltof magnesium to cause precipitation of iron and zirconium and theformation of another heel in said second melt adapted to be charged intoa third melt to remove iron impurity from said third melt.

7. In combination with the method of removing iron from a body of moltenmagnesium by charging. at least about 0.25 per cent of zirconiumchloride into said body of molten magnesium to cause precipitation ofiron and zirconium and i'ormation of a residue containing saidprecipitate-containing portion of the resultant melt to provide a heelcomprising said precipitate-containing portion adapted to be mixed inyet another body of molten magnesium to provide zirconium and removeiron in that body of magnesium.

PHILIPVT. STROUP. GEORGE F. SAGER.

